Viruses are classified on the basis of their nucleic acid content, their size, the shape of their protein capsid, and the presence of a surrounding lipoprotein envelope.

The primary taxonomic division is into two classes based on nucleic acid content: DNA viruses or RNA viruses. The DNA viruses are subdivided into those that contain either double-stranded or single-stranded DNA. The RNA viruses also are divided into those that contain double-stranded or single-stranded RNA. Further subdivision of the RNA viruses is based on whether the RNA genome is segmentedor not. If the viruses contain single-stranded RNA as their genetic information, they are divided into positive-strand viruses if the RNA is of messenger sense (directly translatable into proteins) or negative-strand viruses if the RNA must be transcribed by a polymerase into mRNA.

All viruses falling into one of these nucleic acid classifications are further subdivided on the basis of whether the nucleocapsid (protein coat and enclosed nucleic acid) assumes a rodlike or a polygonal (usually icosahedral) shape. The icosahedral viruses are further subdivided into families based on the basis of the number of capsomeres making up the capsids. Finally, all viruses fall into two classes depending on whether the nucleocapsid is surrounded by a lipoprotein envelope.

Some virologists adhere to a division of viruses into those that infect bacteria, plants, or animals; these classifications have some validity, particularly for the unique bacterial viruses with tails, but there is otherwise so much overlap that taxonomy based on hosts seems unworkable. Classification based on diseases caused by viruses also is not tenable, because closely related viruses frequently do not cause the same disease. Eventually, it is likely that the classification of viruses will be based on their nucleotide sequences and their mode of replication , rather than on structural components, as is now the case.

The basic taxonomic group is called a family, designated by the suffix -viridae. The major taxonomic disagreement among virologists is whether to segregate viruses within a family into a specific genus and further subdivide them into species names. Most virologists believe that a In the first decade of the 21st century, there occurred a shift toward the use of binomial nomenclature, as used for bacteria, dividing viruses into italicized and Latinized genera and species is unwarranted. For this reason, no Latin names are used in the classification of viruses presented here. Moreover, only the well-characterized viruses of animals are presentedgenera and species. This move was prompted in large part by the International Committee on Taxonomy of Viruses (ICTV), a member group of the International Union of Microbiological Societies. The ICTV oversees the ongoing process of devising and maintaining a universal classification scheme for viruses. In the virus classification hierarchy, the ICTV recognizes orders, families, subfamilies, genera, and species. The placement of viruses in these groups is based on information provided by study groups composed of experts on specific types of viruses.

In the ICTV system, each species of virus is generally recognized as representing a group of isolates, or viruses with distinct nucleic acid sequences. Thus, a single species of virus may sometimes contain more than one isolate. Although the isolates of a species possess unique genetic sequences, they all descend from the same replicating lineage and therefore share particular genetic traits. Furthermore, isolates of a species also share in common the ability to thrive within a specific ecological niche. As scientists identify new isolates and species, the classification of viruses is expected to become increasingly complex. The following scheme presents examples of well-characterized DNA and RNA viruses as they are classified on the basis of the ICTV system.

Annotated classificationDNA virusesFamily PoxviridaeLarge viruses of complex structure with dimensions of 400 × 250 nm, the genome of which is linear double-stranded DNA. Virions contain at least 40 proteins and lipids, as well as internal structures called lateral bodies. The 2 subfamilies are called Chordopoxvirinae, which infect vertebrates and are closely related antigenically, and Entomopoxvirinae, which infect arthropods. The Chordopoxvirinae are composed of groups called orthopoxviruses (vaccinia), parapoxviruses, avipoxviruses of birds, and many others that infect sheep, rabbits, and swine.Family AdenoviridaeNonenveloped virions of icosahedral symmetry, about 80 nm in diameter, and capsids containing 252 capsomeres with 12 vertices to which are attached glycoprotein fibres 10–30 nm in length with knobs at the ends. The genome is linear double-stranded DNA. Classified in 2 subgroups: mastadenoviruses, which infect mammals, and aviadenoviruses, which infect birds. Common acute respiratory and gastrointestinal pathogens of humans, and some types cause malignant transformation of cultured cells and can cause cancer in animals.Family HerpesviridaeIcosahedral virions with capsid about 105 150–200 nm in diameter and 162 capsomeres surrounded by a floppy envelope containing glycoprotein spikes. Genome composed of linear double-stranded DNA. There are 3 known subfamilies: Alphaherpesvirinae, consisting of human herpes simplex viruses types 1 and 2, bovine mamillitis virus, SA8 virus and monkey B virus, pseudorabies virus, equine herpesvirus, and varicella-zoster virus; Betaherpesvirinae, composed of species of cytomegaloviruses; and Gammaherpesvirinae, composed of genera familiarly called Epstein-Barr virus, baboon herpesvirus, chimpanzee herpesvirus, Marek’s disease virus of chickens, turkey herpesvirus, herpesvirus saimiri, and herpesvirus ateles.Family IridoviridaeLarge enveloped or nonenveloped icosahedral virions measuring 130–150 120–350 nm in diameter and containing linear double-stranded DNA. Genera include iridovirus and chloriridovirus, which infect insects and contain the Iridovirus, which contains invertebrate iridescent virus 6, and Lymphocystivirus, which contains lymphocystis disease virus 1 of fish.Family AsfarviridaeIcosahedral, enveloped virions approximately 200–300 175–215 nm in diameter that contain linear double-stranded DNA. This family consists of one genus, asfivirusAsfivirus, which contains the African swine fever virus.Family HepadnaviridaeSmall enveloped, spherical virions about 42 40–48 nm in diameter containing circular double-stranded DNA with a single-stranded DNA region and a DNA-dependent DNA polymerase that repairs the single-stranded DNA gap and is essential for replication. Also characteristic are the use of reverse transcriptase for replication and an abundance of a soluble protein (HBsAg). Genera include human Orthohepadnavirus, which consists of hepatitis B virus, woodchuck hepatitis virus, ground squirrel hepatitis virus, duck hepatitis virus, and several others thus far less well documented.Family PapovaviridaeIcosahedral, nonenveloped virions with 42 capsomeres comprising 2 genera: the polyomaviruses (SV40 and polyomavirus), measuring 45 nm in diameter; and the papillomaviruses, measuring 55 nm in diameter. Virions of both genera viruses that infect mammals, and Avihepadnavirus, which consists of hepatitis B viruses that infect birds.Family PapillomaviridaeIcosahedral, nonenveloped virions about 52–55 nm in diameter with 72 capsomeres. Virions contain covalently linked circular DNA. The polyomaviruses produce malignant transformation of infected cells, whereas the papillomaviruses, which Papillomaviruses do not grow in cell culture, and they usually cause warts and benign papillomas, which can also lead to cancer.; in some instances papillomas develop into cancers. The family contains multiple genera. Family ParvoviridaeSmall icosahedral, nonenveloped virions with 32 capsomeres measuring 18–26 nm in diameter that contain single-stranded DNA. Viruses of this family are divided into two subfamilies: Parvovirinae, which infect vertebrates, and Densovirinae, which infect insects. The vertebrate viruses fall into 2 classes: those that replicate autonomously and those that replicate only in the presence of helper adenoviruses or herpesviruses, designated adenoassociated viruses (AAV).Family PolyomaviridaeIcosahedral, nonenveloped virions 40–55 nm in diameter. Virions contain covalently linked circular double-stranded DNA. The family consists of one genus, Polyomavirus. The polyomaviruses produce malignant transformation of infected cells.RNA virusesFamily PicornaviridaeSmall icosahedral, nonenveloped virions 20–30 nm in diameter, composed of 60 capsomeres and containing nonsegmented single-stranded, positive-strand sense RNA. There are 4 Among the multiple recognized genera designated enteroviruses are Enterovirus(polioviruses, Coxsackie viruses, echoviruses), cardiovirusesCardiovirus, rhinoviruses Rhinovirus(common cold viruses), and aphthoviruses Aphthovirus(foot-and-mouth disease virusof cattle).Family CaliciviridaeIcosahedral, nonenveloped virions about 38 35–39 nm in diameter, composed of 32 capsomeres and 180 molecules of a single capsid protein. The genome consists of a single strand of positive-strand sense RNA. The prototype virus of this family is the vesicular exanthema of swine virus.Family TogaviridaeSmall enveloped TogaviridaeEnveloped virions spherical in shape with icosahedral nucleocapsid of 32 capsomeres about 30 70 nm in diameter. The genome is single-stranded positive-strand sense RNA. There are 2 recognized genera: 1 Alphavirus and Rubivirus. Alphavirus consists of viruses transmitted by arthropods (exclusively mosquitoes)and designated alphaviruses (; prototypes include Sindbis virus , and eastern and western equine encephalitis viruses), the other . Rubivirus contains non-arthropod-borne and designated rubivirus (viruses, including the causative agent of German measles).Family FlaviviridaeViruses of this family are enveloped and spherical in shape, with a genome consisting of nonsegmented single-stranded positive-strand sense RNA. These viruses are transmitted by either insects or arachnids and cause severe diseases such as yellow fever, dengue, tick-borne encephalitis, and Japanese B encephalitis. Recently characterized Other members of this family are include non-arthropod-borne hog cholera virus (pestivirus) and hepatitis C virus of humans.Family CoronaviridaeEnveloped virions 60–220 120 nm in diameter with a helical nucleocapsid containing a single strand of positive-strand sense RNAgenome. Club-shaped glycoprotein spikes in envelope give crownlike (coronal) appearance. Viruses of this family are important agents of gastrointestinal disease in humans, poultry, and bovines.Family OrthomyxoviridaeEnveloped virions about 60 to 300 80–120 nm in diameter with a helical nucleocapsid containing 8 segments of single-stranded negative-strand sense RNA and endogenous RNA polymerase. The lipoprotein envelope contains 2 glycoproteins, designated hemagglutinin (major antigen) and neuraminidase. The only best-known viruses in this family are influenza viruses of the 3 distinct antigenic types (of influenza viruses: A, B, and C).Family ParamyxoviridaeEnveloped virions varying in size from 150 to 300 200 nm in diameter with a helical nucleocapsid containing a single strand of negative-strand sense nonsegmented RNA and an endogenous RNA polymerase. The lipoprotein envelope contains 2 glycoprotein spikes designated hemagglutinin-neuraminidase (HN) and fusion factor (F). The major genus is paramyxovirus and is composed of subfamily is Paramyxovirinae, which contains the human parainfluenza viruses and mumps virus, as well as Newcastle disease virus of poultry. The genus morbillivirus Morbillivirus, within Paramyxovirinae, contains the agents that cause measles in humans, distemper in dogs and cats, and rinderpest in cattle. The third genussecond subfamily, pneumovirusPneumovirinae, causes the serious respiratory syncytial virus disease in human infantsand is also classified in the subfamily Pneumovirinae.Family RhabdoviridaeEnveloped virions, usually bullet-shaped, 180–300 × 65 nmabout 75 nm in diameter and 180 nm in length, containing a helical nucleocapsid with single-stranded negative-strand sense RNA and an endogenous RNA polymerase. The lipoprotein envelope contains a single glycoprotein, which is the type-specific antigen. Viruses of this family are widely infectious for plants and for animals varying from insects to humans. 2 animal genera are designated vesiculovirusGenera that infect animals are Vesiculovirus, which includes the virus that causes vesicular stomatitis in cattle, swine, and equines, and lyssavirusLyssavirus, which includes the causative agent of rabies.Family FiloviridaeEnveloped virions, variably elongated filaments 650–14650–1,000 400 nm in length and pleomorphic in shape, containing a helical nucleocapsid with single-stranded negative-strand sense RNA (about 19 kilobases in length) and an endogenous RNA polymerase. Much like the Rhabdoviridae, the lipoprotein envelope contains a single glycoprotein, which is the type-specific antigen. The family consists of 1 genus, filovirus, that 2 genera: Filovirus, which contains the Marburg viruses, and Ebolavirus, which contains the Ebola viruses. These viruses have been isolated from African monkeys, and both are among the most dangerous pathogens. Some strains cause severe hemorrhagic fevers in humans; the mortality rate from these diseases is as high as 88 90 percent. Human infections with Marburg virus have been traced to laboratory monkeys, but human outbreaks of fatal Ebola virus infection in Congo (Kinshasa) and The Sudan have not been traced to monkeys. Instead, these infections are suspected to have been transmitted from fruit-eating bats.Family ArenaviridaeEnveloped virions 100–200 110–130 nm in diameter with a helical nucleocapsid in 2 segments containing negative-strand sense RNA, an endogenous RNA polymerase, and small amounts of ribosomal RNA. There are 4 genera, with viruses The family contains a single genus, Arenavirus, with species widely distributed in animals and causing serious human diseases, many . Many of these agents are transmitted by insects.Family BunyaviridaeEnveloped virions about 95 80–120 nm in diameter with a 3-segment helical nucleocapsid containing single-stranded RNA of negative sense and endogenous RNA polymerase. Many viruses (about 350 species) grouped in 5 genera: bunyavirusOrthobunyavirus, phlebovirusPhlebovirus, nairovirusNairovirus, tospovirusTospovirus, and hantavirusHantavirus. Most of these viruses are transmitted by arthropods and cause serious human disease.Family RetroviridaeEnveloped virions about 90 80–100 nm in diameter with 2 identical copies of single positive-strand RNA in nondefective virions and a reverse transcriptase, which promotes synthesis of double-stranded DNA from the viral RNA template. A hallmark of the virion RNA templates is long terminal repeat (LTR) nucleotide sequences, which serve for integration of the DNA in chromosomes of the host cell. Retroviridae cause cancers in many species of animals, including humans, and are probably derived from normal cell nucleotide sequences called proto-oncogenes. Certain retroviruses of the lentivirus group cause AIDS in humans, monkeys, felines, and cattle.Family ReoviridaeNonenveloped icosahedral virions with outer and inner protein shells 60–80 nm in diameter and containing double-stranded RNA in 10 to 12 segments. Viruses in this family infect many species of plants and animals. The animal Reoviridae are divided into 4 genera, designated orthoreoviruses, orbiviruses genera containing species known to infect animals include Orthoreovirus, Orbivirus(widely distributed in insects and vertebrates, including bluetongue disease virus of sheep), rotaviruses Rotavirus(widespread causative agents of gastroenteritis in mammals, including humans), and cypovirus Cypovirus(prototype causes cytoplasmic polyhedrosis disease in insects).